The present invention relates to probes for use in coordinate measuring machines (CMMs) or machine tools, and has particular reference to a probe with a stylus for contacting the surface of a workpiece.
Probes for use in machine tools and coordinate measuring machines, are mounted on the end of a spindle of a machine tool, or a quill of a CMM, and are moved towards a workpiece to bring a stylus, which projects from the base of the probe, into contact with a surface of a workpiece. Contact between the stylus and the surface of the workpiece is indicated by a signal from the signalling mechanism within the probe, which causes the machine to stop and the machine scale readings to be latched. By contacting a number of points on the surface of a workpiece, the shape and/or dimensions of the workpiece or a feature on the workpiece can be determined.
The accuracy of the measuring process is dependent upon the probe repeatably generating a signal each time the stylus tip is at a predetermined position relative to the probe body. However, vibrations of the quill or spindle upon which the probe is mounted, if transmitted to the probe stylus, give rise to an uncertainty in the position of the probe stylus tip and the repeatability of the signalling is degraded. If the vibrations transmitted to the stylus happen to coincide with a natural resonant frequency of the stylus or the combination of the stylus and its mounting, the amplitude of the vibration at the stylus tip can be relatively large and give rise to significant errors in the subsequent measurements on the workpiece.
In the past it has been known to change one or more characteristics of the stylus e.g. its material or diameter, to change its stiffness such that its resonant frequency is moved away from the predominant forcing frequency of the vibration of the machine spindle. This is preferably achieved by increasing the resonant frequency of the stylus beyond the predominant forcing frequency of the machine.
The effects of simply changing the resonant frequency of the stylus are limited in the extent to which they can change the amplitude of the vibration of the stylus at its resonant frequency, or at other frequencies, and do not therefore always provide a satisfactory solution to the problem.
The present invention provides an alternative method of changing the amplitude of the vibration of a stylus at its resonant frequency which is more effective than the above-described previous methods.
In accordance with the present invention, in its broadest application a stylus of a measuring probe has an internal cavity in which is provided vibration damping means.
In a first embodiment of the invention the vibration damping means comprises an unconstrained material mass.
A variety of materials have been tried ranging from rubber sleeves of different diameters, glass dust, and finely chopped wire. Each produced a significant reduction in the maximum amplitude of stylus vibration with varying beneficial effects on other vibration amplitudes.
In a second embodiment of the invention the vibration damping means comprises a dynamic vibration absorber which is tuned to the natural frequency of vibration of the stylus.
In both embodiments the vibration damping means is preferably provided adjacent the tip of the stylus where the amplitude of the vibration is maximum.